>(The speed is about the same.) I'd imagine it's cost. Both system consist of booster (which need to provide 20 Mach delta-v to its payload) and payload - uppe

>(The speed is about the same.) I'd imagine it's cost. Both system consist of booster (which need to provide 20 Mach delta-v to its payload) and payload - upper stage which is warhead carrying vehicle. ICBM is cheaper because it follows dumb ballistic trajectory which puts no specific structural requirements on the warheads and their enclosure. The warhead vehicle traveling at 20 Mach in the upper atmosphere would cost enormously more ( and possibly would be much heavier, thus requiring bigger booster). I.e. ICBM is cheaper. The dumb ballistic trajectory was a weak spot of the ICBM system, and thus modern ICBM warheads perform evasive maneuvers during final atmosphere re-entry.